plugin/forward: add hostname resolution support for TO endpoints (#5646) (#7923)

Signed-off-by: Dmytro Alieksieiev <1865999+dragoangel@users.noreply.github.com>

plugin/forward/setup.go

@@ -4,6 +4,7 @@ import (
 	"crypto/tls"
 	"errors"
 	"fmt"
+	"net"
 	"path/filepath"
 	"strconv"
 	"strings"
@@ -32,8 +33,8 @@ func setup(c *caddy.Controller) error {
 	}
 	for i := range fs {
 		f := fs[i]
-		if f.Len() > max {
-			return plugin.Error("forward", fmt.Errorf("more than %d TOs configured: %d", max, f.Len()))
+		if len(f.toEntries) > max {
+			return plugin.Error("forward", fmt.Errorf("more than %d TOs configured: %d", max, len(f.toEntries)))
 		}
 
 		if i == len(fs)-1 {
@@ -146,11 +147,7 @@ func parseStanza(c *caddy.Controller) (*Forward, error) {
 		return f, c.ArgErr()
 	}
 
-	toHosts, err := parse.HostPortOrFile(to...)
-	if err != nil {
-		return f, err
-	}
-
+	// Parse block first to get resolver and other options before processing TO addresses.
 	for c.NextBlock() {
 		if err := parseBlock(c, f); err != nil {
 			return f, err
@@ -161,6 +158,22 @@ func parseStanza(c *caddy.Controller) (*Forward, error) {
 		return f, fmt.Errorf("max_age (%s) must not be less than expire (%s)", f.maxAge, f.expire)
 	}
 
+	// Classify TO addresses in order, preserving config ordering.
+	entries, err := classifyToAddrs(to)
+	if err != nil {
+		return f, err
+	}
+	f.toEntries = entries
+
+	// Expand hostnames and deduplicate globally (first-seen order wins).
+	toHosts, err := expandAndDedup(f.toEntries, f.resolver)
+	if err != nil {
+		return f, err
+	}
+	if len(toHosts) == 0 {
+		return f, fmt.Errorf("no valid upstream addresses found")
+	}
+
 	tlsServerNames := make([]string, len(toHosts))
 	perServerNameProxyCount := make(map[string]int)
 	transports := make([]string, len(toHosts))
@@ -424,6 +437,21 @@ func parseBlock(c *caddy.Controller, f *Forward) error {
 
 			f.failoverRcodes = append(f.failoverRcodes, rc)
 		}
+	case "resolver":
+		args := c.RemainingArgs()
+		if len(args) == 0 {
+			return c.ArgErr()
+		}
+		for _, arg := range args {
+			host := arg
+			if h, _, err := net.SplitHostPort(arg); err == nil {
+				host = h
+			}
+			if net.ParseIP(host) == nil {
+				return fmt.Errorf("resolver must be an IP address or IP:port: %q", arg)
+			}
+		}
+		f.resolver = args
 	default:
 		return c.Errf("unknown property '%s'", c.Val())
 	}